Presentation is loading. Please wait.

Presentation is loading. Please wait.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece.

Similar presentations


Presentation on theme: "Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece."— Presentation transcript:

1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 14 Mendel and the Gene Idea

2 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Points to Ponder What is the genotype and the phenotype of an individual? What are the genotypes for a homozygous recessive and dominant individuals and a heterozygote individual? Be able to draw a punnett square for any cross (1-trait cross, 2-trait cross and a sex-linked cross). What are Tay-Sachs disease, Huntington disease, sickle-cell disease, and PKU? How are each of the above inherited? What is polygenic inheritance? What is a multifactorial trait? What is sex-linked inheritance? Name 3 X-linked recessive disorders. What is codominance? What is incomplete dominance? What do you think about genetic profiling?

3 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings These traits are genetically inherited Answer these questions about your inheritance: Do you have a widows peak? Are your earlobes attached or unattached? Do you have short or long fingers? Do you have freckles? Can you roll your tongue? Do you have Hitchhikers thumb? Genotype and phenotype

4 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Some genetic vocabulary – Gene: the basic unit of heredity in a living organism. A section of a chromosome that codes for a trait, or characteristic. – Alleles: alternate forms of a specific gene at the same position (locus) on a gene (e.g. allele for unattached earlobes and attached lobes); alleles occur in pairs

5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings – Dominant allele: will be expressed and will mask a recessive allele (Tt or TT) – Recessive allele: allele that is only expressed when a gene has two of this type of allele – Homozygous dominant genotype: 2 dominant alleles (TT or AA) – Homozygous recessive genotype: 2 recessive alleles (tt or aa) – Heterozygous genotype: one dominant allele and one recessive allele (Tt or Aa) Genotype and phenotype

6 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genotype – specific genes for a particular trait written with symbols Phenotype – the physical or outward expression of the genotype Genotype Phenotype EE unattached earlobe Ee unattached earlobe ee attached earlobe What are your genotype and phenotype? Genotype and phenotype

7 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Understanding genotype & phenotype Genotype and phenotype Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. sperm egg fertilization growth and development unattached earlobeattached earlobeunattached earlobe Ee e ee EE E eeEE EeeeEE Allele Key E= unattached earlobes e= attached earlobes zygote

8 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gregor Mendel Documented a particulate mechanism of inheritance through his experiments with garden peas Figure 14.1

9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings particulate inheritance – Parents pass on discrete heritable units, genes (Mendel referred to these as factors)

10 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendels experiments Concept 14.1: Mendel used the scientific approach to identify some basic laws of inheritance Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned and executed experiments

11 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendels Experimental, Quantitative Approach Mendel chose to work with peas – Because they are available in many varieties – Because he could strictly control which plants mated with which

12 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendels experiments : Controls Wisely, Mendel chose to track only those characters that varied in an either-or manner: - dominant or recessive, no blended characteristics, no polygenic traits Mendel also made sure that: – He started his experiments with varieties that were true-breeding (purebred lines isolated through self-pollination)

13 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In a typical breeding experiment – Mendel mated two contrasting, true-breeding varieties, a process called hybridization The true-breeding parents are called the P generation The hybrid offspring of the P generation are called the F 1 generation When F 1 individuals self-pollinate, the F 2 generation is produced

14 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Law of Segregation When Mendel crossed contrasting, true-breeding white and purple flowered pea plants – ALL of the offspring were purple. Why?

15 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Useful Genetic Vocabulary An organism that is homozygous for a particular gene – Has a pair of identical alleles for that gene (ex: PP or pp) – Exhibits true-breeding An organism that is heterozygous for a particular gene (example: Pp) – Has a pair of alleles that are different for that gene (hybrid)

16 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings …but when Mendel crossed the F 1 plants: – Many of the F 2 plants had purple flowers, but some had white flowers – He found a repeatable ratio of about three to one, purple to white flowers, in the F 2 generation Figure 14.3 P Generation (true-breeding parents) Purple flowers White flowers F 1 Generation (hybrids) All plants had purple flowers F 2 Generation EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by ). The resulting F 1 hybrids were allowed to self-pollinate or were cross- pollinated with other F 1 hybrids. Flower color was then observed in the F 2 generation. RESULTS Both purple-flowered plants and white- flowered plants appeared in the F 2 generation. In Mendels experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white.

17 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendels Factors (genes) Mendel reasoned that – In the F 1 plants, only the purple flower factor was affecting flower color in these hybrids – Purple flower color was dominant, and white flower color was recessive In such cases, the dominant allele MASKS the recessive

18 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendel observed the same pattern in many other pea plant characters Mendel developed a hypothesis to explain the 3:1 inheritance pattern that he observed among the F 2 offspring Four related concepts make up this model… Table 14.1

19 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Alleles First, alternative versions of genes – Account for variations in inherited characters, which are now called alleles Figure 14.4 Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers

20 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Second, for each character – An organism inherits two alleles, one from each parent – A genetic locus is actually represented twice Figure 14.4 Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers Homologous chromosomes, one paternal, one maternal

21 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Dominants and Recessives Third, if the two alleles at a locus differ – Then one, the dominant allele, determines the organisms appearance – The other allele, the recessive allele, has no noticeable effect on the organisms appearance (is masked)

22 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Vocabulary review Allele: different forms of a gene (for a trait). Homozygous: purebred; identical set of alleles for a trait Heterozygous: hybrid; nonmatching alleles

23 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Segregation principle Fourth, the law of segregation – The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

24 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Punnett Squares Does Mendels segregation model account for the 3:1 ratio he observed in the F 2 generation of his numerous crosses? The Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment. It is named after Reginald C. Punnett, who devised the approach, and is used by biologists to determine the probability of an offspring having a particular genotype. Reginald C. Punnett biologists probabilitygenotype

25 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Possible gametes for two traits 20.2 One-and Two-trait inheritance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. MEIOSIS I MEIOSIS II eitheror one pair SS WW ss w w WW ww WW WWWW WswSwsWS wwww WWwwww ss sss ss ss s SS S SSSS SSS Cell has two pairs of homologues. Allele Key = = = = W w S s Widows peak Straight hairline Short fingers Long fingers

26 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings YYRR P Generation GametesYRyr yyrr YyRr Hypothesis of dependent assortment Hypothesis of independent assortment F 2 Generation (predicted offspring) 1 2 YR yr 1 2 yr YYRRYyRr yyrr YyRr Sperm Eggs Phenotypic ratio 3:1 YR 1 4 Yr 1 4 yR 1 4 yr YYRR YYRr YyRR YyRr YyrrYyRr YYrr YyRR YyRr yyRRyyRr yyrr yyRr Yyrr YyRr Phenotypic ratio 9:3:3: Phenotypic ratio approximately 9:3:3:1 F 1 Generation Eggs YR Yr yRyr 1 4 Sperm RESULTS CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. EXPERIMENT Two true-breeding pea plants one with yellow-round seeds and the other with green-wrinkled seedswere crossed, producing dihybrid F 1 plants. Self-pollination of the F 1 dihybrids, which are heterozygous for both characters, produced the F 2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant. A dihybrid cross – Illustrates the inheritance of two characters Produces four phenotypes in the F 2 generation Figure 14.8

27 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Independent Assortment Principle Using the information from a dihybrid cross, Mendel developed the law of independent assortment – Each pair of alleles segregates independently during gamete formation – Animated version:

28 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The reality of inheritance Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics The relationship between genotype and phenotype is rarely simple What about green eyes? Hazel eyes?

29 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Extending Mendelian Genetics for a Single Gene The inheritance of characters by a single gene – May deviate from simple Mendelian patterns – Exceptions to the rules are listed in the following slides Need practice? Try these drag and drop Punnett squares

30 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The extent of variation Not only does meiosis guarantee segregation and independent assortment of alleles, but crossing over also mixes up alleles on homologous chromosomes before distribution Therefore, in humans with 23 pairs of chromosomes, a gamete (egg or sperm) could have 2 23 or 8,388,604 possible combinations of chromosomes from that parent. Any couple could have 2 23 × 2 23 or 70,368,744,177,644 (70 trillion) different possible children, based just on the number of chromosomes, not considering the actual genes on those chromosomes. Thus, the chance of two siblings being exactly identical would be 1 in 70 trillion. In addition, something called crossing over, in which the two homologous chromosomes of a pair exchange equal segments during synapsis in Meiosis I, can add further variation to an individuals genetic make-up.

31 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sex and variation Crossing over prior to gamete formation increases variables Independent assortment Crossover animation

32 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Spectrum of Dominance Complete dominance – Occurs when the phenotypes of the heterozygote and dominant homozygote are identical PP Pp pp

33 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Incomplete dominance The phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties Figure P Generation F 1 Generation F 2 Generation Red C R Gametes CRCR CWCW White C W Pink C R C W Sperm CRCR CRCR CRCR CwCw CRCR CRCR Gametes 1 2 Eggs 1 2 C R C R C W C W C R C W Example: Flower colors RR = red RR = white RR = pink (intermediate pigmentation)

34 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Spectrum of Dominance In codominance – Two dominant alleles affect the phenotype in separate, distinguishable ways I A I B = The human blood group type AB is an example of codominance RW = roan coat color in cattle or horses

35 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Multiple Alleles and Codominance The ABO blood group in humans – Is determined by multiple alleles Table 14.2 Most genes exist in populations – In more than two allelic forms

36 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Frequency of Dominant Alleles Dominant alleles – Are not necessarily more common in populations than recessive alleles – It really depends on whether a trait gives an individual an adaptive advantage, and is naturally selected. – Examples: type O blood (ii) recessive present in majority of population

37 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pleiotropy Pleiotropy describes the genetic effect of a single gene on multiple phenotypic traits. The underlying mechanism is that the gene codes for a product that is, for example, used by various cells, or has a signaling function on various targets. A classic example of pleiotropy is the human disease PKU (phenylketonuria).phenylketonuria This disease can cause mental retardation and reduced hair and skin pigmentation, and can be caused by any of a large number of mutations in a single gene that codes for the enzyme (phenylalanine hydroxylase), which converts the amino acid phenylalanine to tyrosine, another amino acid.mental retardationhairskin pigmentationphenylalanine hydroxylasephenylalaninetyrosine Depending on the mutation involved, this results in reduced or zero conversion of phenylalanine to tyrosine, and phenylalanine concentrations increase to toxic levels, causing damage at several locations in the body. PKU is totally benign if a diet free from phenylalanine is maintained.

38 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene Linkage Sometimes, when genes are located close together on the same chromosome, they tend to be inherited together. Which of Mendels Laws does this defy? See linkage animation A classic example of this is why cats with white fur and blue eyes are (more often than not) also deaf.

39 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Extending Mendelian Genetics for Two or More Genes Many traits (especially in complex organisms) – May be determined by two or more genes (polygenic) In EPISTASIS standing upon – the phenomenon where the effects of one gene are modified by one or several other genes (which are sometimes called modifier genes). – A gene at one locus alters the phenotypic expression of a gene at a second locus

40 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An example of epistasis in mice Figure BC bCBc bc 1 4 BC bC Bc bc 1 4 BBCcBbCc BBcc Bbcc bbcc bbCc BbCc BbCC bbCC BbCc bbCc BBCCBbCC BBCc BbCc BbCc Sperm Eggs Here we see the effect the bb gene combo has on the C gene for the agouti (brownish) fur color in mice cc combo = white fur C_ combo = black fur, unless bb stands upon it. Then you get agouti color.

41 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Other variations in gene expression Incomplete Expressivity is seen in cases where individuals with the same genotypes may have, often for unknown reasons, variability in their phenotypes. – This is often seen in genetic diseases where one person with a disease such as diabetes may be very severely effected while another with the same allele may have a milder form of the disease. Incomplete Penetrance is seen when an individual with a particular genotype does not express the phenotype. Again the reasons for this are not clearly understood. – For example, known mutations in the gene responsible for Huntington disease have 95% penetrance, because 5% of those with the dominant allele for Huntington disease don't develop the disease and 95% do.mutations Huntington disease

42 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Polygenic traits - two or more sets of alleles govern one trait – Each dominant allele codes for a product so these effects are additive – Results in a continuous variation of phenotypes – e.g. skin color ranges from very dark to very light Beyond simple inheritance AaBbCc aabbcc Aabbcc AaBbccAaBbCcAABbCc AABBCcAABBCC Fraction of progeny Note that: Environmental effects can cause intervening phenotypes!

43 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Polygenic inheritance Courtesy University of Connecticut/Peter Morenus, photographer; Height in Inches shorttall most are this height few Number of Men Multifactorial trait – a trait that is influenced by both genetic and environmental factors e.g. skin color is influenced by sun exposure e.g. height can be affected by nutrition

44 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genes are affected by Environment While there is a strong genetic component in human height, the average height has increased over the past 50 years in developed countries. This is considered to be due to improved nutrition. Likewise, a cotton plant may have the alleles necessary for high yields but if it doesn't receive enough water or fertilizer it cannot reach its genetic potential.

45 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Example: – A phenotypic range of a particular genotype may be influenced by the environment Figure Exact color often mirrors the pH of the soil; acidic soils produce blue flowers, neutral soils produce very pale cream petals, and alkaline soils results in pink or purple. This is caused by a color change of the flower pigments in the presence of aluminum ions which can be taken up into the flower.

46 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Demonstrating environmental influences on phenotype Himalayan rabbit/Siamese cat coat color influenced by temperature There is an allele responsible for melanin production that appears to be active only at lower temperatures The extremities have a lower temperature and thus the ears, nose paws and tail are dark in color Beyond simple inheritance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © H. Reinhard/Arco Images/ Peter Arnold

47 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Humans are not convenient subjects for genetic research. Why? – However, the study of human genetics continues to advance Many human characters – Vary in the population along a continuum and are called quantitative characters

48 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A pedigree – Is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can be traced and described using pedigrees Ww ww Ww wwWw ww Ww WW or Ww ww First generation (grandparents) Second generation (parents plus aunts and uncles) Third generation (two sisters) Ff ffFf ff Ff ff FF or Ff ff FF or Ff Widows peak No Widows peak Attached earlobe Free earlobe (a) Dominant trait (widows peak ) (b) Recessive trait (attached earlobe)

49 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pedigree Analysis Human geneticists illustrate the inheritance of a gene within a family by using a pedigree chart. On such a chart, males are symbolized by a square ( ) and females are symbolized by a circle ( ). People who are affected by a disease are symbolized by a dark circle or square. The pedigree chart below shows inheritance of the gene that causes albinism. A and B represent a couple who had five children, including C and E. Only one of the children, E, was albino. E and her husband had five children, including G. In the pedigree below write the genotypes of the individuals who are labeled with letters, using (A) to represent the dominant allele and (a) to represent the recessive allele. Start by indicating the genotypes of E and F. Then use a Punnett Square to figure out what the genotypes for C and D must be. Next, determine the genotypes of A and B. Finally, determine the genotype of G.

50 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Types of Gene Mutations Point mutations: caused by single-base substitution

51 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sickle-Cell Disease – Affects one out of 400 African-Americans – Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells (shows seven out of the 146 amino acid units of normal hemoglobin)

52 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Electrophoresis of the hemoglobin protein shows: We see that homozygous normal people have one type of hemoglobin (A) and anemics have type S, which moves more slowly in the electric field. The heterozygotes have both types, A and S. In other words, there is codominance at the molecular level. Try this pedigree of familial inheritance of SC diseasepedigree

53 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sickle Cell Disease Homozygous recessive individuals have sickle cell disease Heterozygous individuals are normal, but exhibit a mild version of the disease; said to carry the trait for SC Homozygous dominant individuals are normal. Genotypes:

54 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sickle Cell Symptoms include physical weakness, pain, organ damage, and even paralysis Sickle-cell disease occurs more commonly in people (or their descendants) from parts of the world such as sub-Saharan Africa, where malaria is or was common, but it also occurs in people of other ethnicities. This is because those with one or two alleles of the sickle cell disease are resistant to malaria since the red blood cells are not conducive to the parasites - in areas where malaria is common there is a survival value in carrying the sickle cell genes.sub-Saharan Africa malaria

55 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Review: Types of Gene Mutations Frame shift mutation: caused by insertion or deletion of a base; usually serious because they throw off the codon reading frame Result: protein is nonfunctional

56 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recessively Inherited Autosomal Disorders Many genetic disorders – Are inherited in a recessive manner (aa) – So, most affected individuals are homozygous (dual inheritance) – AA, Aa are normal phenotype, in this case Recessively inherited disorders – Show up only in individuals homozygous for the allele Carriers – Are heterozygous individuals who carry the recessive allele but are phenotypically normal

57 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cystic Fibrosis Symptoms of cystic fibrosis include – Mucus buildup in the some internal organs – Abnormal absorption of nutrients in the small intestine Lung tissue from a cystic fibrosis patient, showing extensive destruction as a result of obstruction and infection. thick mucus defective channel C l - H2OH2O nebulizer percussion vest

58 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Phenyketonuria In 'classic PKU', the enzyme (phenylalanine hydroxylase), that breaks down phenylalanine is completely or nearly completely deficient. This enzyme normally converts phenylalanine to another amino acid, tyrosine. Without this enzyme, phenylalanine and its' breakdown chemicals from other enzyme routes, accumulate in the blood and body tissues. Infants are born normal but if not treated, severe brain problems, such as mental retardation and seizures, will occur. Thus, this disease is GREATLY influenced by environmental factors PKU is an autosomal recessive genetic disorder that is characterized by an inability of the body to utilize the essential amino acid, phenylalanine. Amino acids are the building blocks for body proteins.

59 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Other autosomal recessive disorders Tay-Sachs disease Albinism Hemochromatosis types 1-3: the most common genetic disease in Europe.Hemochromatosisdisease in Europe You can watch this animation:

60 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Dominantly Inherited Disorders Some human disorders – Are due to dominant alleles One example is achondroplasia – A form of dwarfism that is lethal when homozygous for the dominant allele (DD) Figure 14.15

61 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Dominant allele Huntingtons disease – Is a degenerative disease of the nervous system – Has no obvious phenotypic effects until about 35 to 40 years of age Lake Maracaibo, Venezuela Village pedigree

62 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings X-linked disorders More often found in males than females because recessive alleles are always expressed Most X-linked disorders are recessive: – Color blindness: most often characterized by red- green color blindness – Duchennes muscular dystrophy (DMD): characterized by wasting of muscles and death by age 20 – Fragile X syndrome: most common cause of inherited mental impairment – Hemophilia: characterized by the absence of particular clotting factors that causes blood to clot very slowly or not at all Sex-linked inheritance

63 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings X-linked disorders Sex-linked inheritance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. X B X B X B Y X B YX b Y X b Y X B X B X B X b X B YX B X b X b X b X b Y Key X B X B =Unaffected female X B X b =Carrier female X b X b =Color-blind female X B Y=Unaffected male X b Y=Color-blind male grandfather daughter grandson X-linked Recessive Disorders More males than females are affected. An affected son can have parents who have the normal phenotype. For a female to have the characteristic, her father must also have it. Her mother must have it or be a carrier. The characteristic often skips a generation from the grandfather to the grandson. If a woman has the characteristic, all of her sons will have it.

64 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mating of Close Relatives Matings between relatives are calledconsanguineous matings Can increase the probability of the appearance of a genetic disease (especially harmful recessive homozygotes)

65 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings X-linked disorders: Hemophilia Sex-linked inheritance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Key Unaffected male Hemophiliac Unaffected female Carrier Queen Victoria VictoriaAlice Victoria Frederick III (Germany) Nicholas II (Russia) Alexander (Earl of Athlone) Alfonso XII (Spain) Frederick (died at 3) Waldemar (died at 56) Henry (died at 4) Alexei (murdered) Rupert (died at 21) Alfonso (died at 31) Gonzalo (died at 20) HenryIreneAlexandra Louis IV (Hesse) Leopold (died at 31) Leopold (died at 32) Prince Henry of Battenberg BeatricePrincess Helena of Waldeck Prince Albert 4 children of 9 are shown 12 children of 26 are shown 6 children of 34 are shown (queen): © Stapleton Collection/Corbis; (prince): © Huton Archive/Getty Images

66 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Multifactorial Disorders Many human diseases – Have both genetic and environment components Examples include – Heart disease and cancer – Try this interactive family pedigree (nicotine addiction)

67 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Testing and Counseling Genetic counselors – Can provide information to prospective parents concerned about a family history for a specific disease

68 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Counseling Based on Mendelian Genetics and Probability Rules Using family histories – Genetic counselors help couples determine the odds that their children will have genetic disorders

69 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Tests for Identifying Carriers For a growing number of diseases – Tests are available that identify carriers and help define the odds more accurately In amniocentesis – The liquid that bathes the fetus is removed and tested In chorionic villus sampling (CVS) – A sample of the placenta is removed and tested Fetal Testing

70 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Prenatal Genetic Screening 70 Genetic Screening (karyotyping) is possible, prenatally weeks Chorionic villus sampling 9-14 weeks

71 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Epigenetics – Functional modifications to the genome that do not involve a change in the nucleotide sequence. – Environmental factors can alter the way our genes are expressed, making even identical twins different. – Examples of such modifications are DNA methylation and histone modification, both of which serve to regulate gene expression without altering the underlying DNA sequence.DNA methylationhistone modificationDNA sequence Concept: Some inheritance patterns are exceptions to the standard chromosome theory /content/epigenetics/

72 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genomic Imprinting What Is Imprinting? For most genes, we inherit two working copies -- one from mom and one from dad. But with imprinted genes, we inherit only one working copy. Depending on the gene, either the copy from mom or the copy from dad is epigenetically silenced. Silencing usually happens through the addition of methyl groups during egg or sperm formation. The epigenetic tags on imprinted genes usually stay put for the life of the organism. But they are reset during egg and sperm formation. Regardless of whether they came from mom or dad, certain genes are always silenced in the egg, and others are always silenced in the sperm.

73 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genomic imprinting Involves the silencing of certain genes that are stamped with an imprint during gamete production (a) A wild-type mouse is homozygous for the normal igf2 allele. Normal Igf2 allele (expressed) Normal Igf2 allele with imprint (not expressed) Paternal chromosome Maternal chromosome Wild-type mouse (normal size) Normal Igf2 allele Paternal Maternal Mutant lgf2 allele Paternal Maternal Dwarf mouse Normal Igf2 allele with imprint Normal size mouse When a normal Igf2 allele is inherited from the father, heterozygous mice grow to normal size. But when a mutant allele is inherited from the father, heterozygous mice have the dwarf phenotype. In mammals the phenotypic effects of certain genes depend on which allele is inherited from the mother and which is inherited from the father

74 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genes located outside the nucleus also have influence Extranuclear genes – Are genes found in organelles in the cytoplasm The inheritance of traits controlled by genes present in the chloroplasts (ctDNA) or mitochondria (mtDNA) – Depends solely on the maternal parent because the zygotes cytoplasm comes from the egg Figure 15.18

75 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings In Humans: Some diseases affecting the muscular and nervous systems are caused by defects in mitochondrial genes that prevent cells from making enough ATP – Examples of mitochondrial DNA diseases:mitochondrial DNA diseases Leber's hereditary optic atrophy - causes progressive visual impairment Leber's hereditary optic atrophy Kearns-Sayre disease Progressive external ophthalmoplegia Myoclonus epilepsy MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes) MELAS – Mitomap - A human mitochondrial genome database Mitomap


Download ppt "Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece."

Similar presentations


Ads by Google